Various methods are known for determining the concentration of specific constituents of a patient's blood. Methods for measuring the concentration of blood constituents which require a blood sample to be taken are known in the prior art. Measuring methods are however also known in which the concentration of blood constituents in blood flowing through a hose line can be measured. These methods are used especially when the blood is flowing through the hose line of an extracorporeal blood circuit in an extracorporeal blood treatment.
In an extracorporeal blood treatment, particularly in a blood treatment with a haemodialysis machine, the monitoring of the concentration of haemoglobin in the patient's blood is important. Frequent measurements of the haemoglobin concentration enable the early detection of the therapeutic effect through a trend analysis and permit early adaptation of the therapeutic measures.
International Patent Publication No. WO 2008/000433 A1 describes a method and a device for determining the concentration of specific blood constituents in a blood-filled, essentially transparent hose line, in particular a hose line of an extracorporeal blood circuit of an extracorporeal blood treatment apparatus. The known method and the known device make it possible in particular to determine the haemoglobin concentration and the haematocrit.
During the measurement, the hose line is clamped between two parallel, plane contact surfaces, so that the hose is deformed at the sides lying opposite one another. The measurement of the haematocrit is based on the transmission or scattering of electromagnetic radiation of a specific wavelength in the blood. With an emitter, the electromagnetic radiation is coupled through the transparent hose line into the blood, whilst with a detector the scattered transmitted electromagnetic radiation is decoupled from the blood. The haematocrit is then determined from the analysis of the decoupled radiation. The concentrations of other blood constituents, for example, creatinin, bilirubin, glucose, etc., can however also be determined with the known scattered-light or transmission or reflection measurement.
The intensity of the decoupled light depends not only on the concentration of the blood constituents, but also on other factors. The concentration of the blood constituents cannot therefore be readily calculated from the analysis of the decoupled light.
German Application No. DE 10 2010 026 723.6 describes that, in the scattered-light or transmission or reflection measurement, a role is played by the properties of the hose line through which the light is coupled and decoupled. The hose line influences the optical measurement, since the hose partially absorbs and scatters the light. A compensation of the influence of the hose line on the determination of the haemoglobin concentration is therefore provided for.
A method for the quantitative determination of dissolved substances by the light scatter of a laser is described in European Patent Application No. EP 0 074 428. The described method permits the measurement of blood constituents, in particular glucose, in suitable parts of the body, e.g., the earlobe. It can be deduced from the publication that difficulties can in principle arise from the changing flow of blood through the earlobe during the measurement. European Patent Application No. EP 0 575 712 describes a spectrophotochemical blood analysis.